Wireless power-supplying systems which supply power using electromagnetic induction, magnetic field resonance, or the like from a power-transmitting device including a power-transmitting coil to a power-receiving device including a power-receiving coil have become known in recent years. In this system, magnetic flux generated by the power-transmitting coil is interlinked with the power-receiving coil, and thereby power is transmitted between the coils. Therefore, efficiency of power transmission (transmission efficiency) is affected by a positional relationship between the power-transmitting coil and the power-receiving coil.
As one application of such a wireless power-supplying system, battery charging of an electric vehicle has drawn attention. In this case, the power-receiving device is installed in a vehicle. However, due to a limit in driving accuracy, it is difficult to park the vehicle accurately at a predetermined position for charging. Therefore, the positional relationship between the power-transmitting coil and the power-receiving coil may be different each time the vehicle is parked. When the power-transmitting coil and the power-receiving coil are misaligned from a desired positional relationship, a coupling coefficient between the coils may change and the transmission efficiency may decrease.
In the related art, a technology which suppresses a decrease in charging efficiency when a positional misalignment between the coils occurs has been proposed (for example, refer to Patent Document 1). A power supply device (power-transmitting device) of Patent Document 1 changes a frequency of AC power supplied to a power-transmitting coil from an inverter circuit (power supply) if transmission efficiency decreases due to occurrence of the positional misalignment. The power supply device further improves the charging efficiency by a change in the frequency.